Point of contact detection

ABSTRACT

The present invention is directed to a process for determining at least one first coordinate of the point of contact of a ball on the strings of a ball game racquet, as well as a ball game racquet suitable for carrying out such a process.

This application claims priority under 35 U.S.C. §119 to German PatentApplication No. DE 10 2014 003 353.8, filed Mar. 7, 2014, the entiretyof which is incorporated herein by reference.

The present invention is directed to a process for determining at leastone coordinate of the point of contact or impact of a ball on thestrings of a ball game racquet, as well as a ball game racquet suitablefor carrying out such a process.

It has been known for a long time that the point of contact of a ball onthe strings of a ball game racquet significantly influences theperformance and efficiency of a player. If the so-called “sweet spot” ofthe racquet is hit, both the transmission of force from the racquet, orits strings, to the ball and the control of the flight direction of theball are optimal. Therefore, attempts were made quite a while ago toprovide practice racquets by means of which it was possible to determineor monitor whether the ball has hit that sweet spot. For example, DE 19816 389 A1 describes a tennis racquet for practicing accuracy andimproving the efficiency of the hit, which has an integrated sensor inits strings. This sensor emits a signal if, and only if, it is hit bythe ball. If the ball makes contact next to the sensor, no signal isgenerated. DE 29 425 33 A1 also describes a tennis racquet with a hitsignal generator which generates a hit signal if the tennis ball hitsthe central area of the strings. However, these tennis racquets have thedisadvantage that the player merely receives a qualitative signal (hitthe sweet spot or did not hit the sweet spot), without getting anyinformation regarding the actual point of contact of the ball on thestrings. U.S. Pat. No. 4,101,132 and U.S. Pat. No. 4,257,594 provide animproved tennis racquet in that several zones can be defined on theracquet and it can be determined by means of several sensors which ofthese zones were hit by the ball. However, this type of tennis racquetalso only generates a discrete signal. Furthermore, due to the largenumber of sensors needed for the increasing number of zones, this tennisracquet becomes technically complex and thus expensive. Finally, EP 0377 614 B1 describes a tennis racquet with a plurality of sensing meanslocated at the periphery of its playing surface to detect shock wavestraversing along the surface when the ball hits the strings. Then, adistinction is made between the points in time when the shock waves areinitially detected by the individual sensing means. If the points intime fall within a predetermined reference time frame which correspondsto the sweet spot, a signal is emitted indicating that the sweet spot ofthe tennis racquet has been hit. However, as is immediately apparent,such a tennis racquet requires an extremely high time resolution if thepoint of contact of the ball is to be determined with high accuracy.Consequently, necessary sensor technology is technically sophisticatedand therefore expensive.

It is therefore an object of the present invention to provide animproved process for determining at least one coordinate of the point ofcontact or impact of a ball on the strings of a ball game racquet whichtakes into account the disadvantages of the processes known in the priorart discussed above. It is furthermore an object of the presentinvention to provide a ball game racquet suitable for carrying out sucha process. These objects are achieved by the process according to claims1 and 2 and by a ball game racquet according to claims 15 and 18.Preferred embodiments of the present invention are described in thedependent claims.

The present invention is directed, inter alia, to a process fordetermining at least one first coordinate of the point of contact orimpact of a ball on the strings of a ball game racquet. The ball gameracquet comprises a racquet head and a handle section, wherein thelongitudinal axis defines an x-coordinate, the transverse axis defines ay-coordinate and the line perpendicular to the strings defines az-coordinate. According to the present invention, at least one kinematicbasic parameter or kinematic variable is measured in a first directionas a function of time at a first point or location of the ball gameracquet, wherein the measurement rate is preferably at least 200 Hz. Theat least first measured kinematic basic parameter or kinematic variableis then transformed into the frequency domain. The first coordinate ofthe point of contact of the ball on the strings of the ball game racquetis calculated on the basis of the transformed kinematic basic parameterin the frequency domain.

Furthermore, the present invention is directed to a process fordetermining a first and a second coordinate of the point of contact orimpact of a ball on the strings of a ball game racquet. The ball gameracquet comprises a racquet head and a handle section, wherein thelongitudinal axis defines an x-coordinate, the transverse axis defines ay-coordinate and the line perpendicular to the strings defines az-coordinate. According to this alternative preferred embodiment of thepresent invention, a first kinematic basic parameter or kinematicvariable is measured in a first direction as a function of time at afirst point or location of the ball game racquet and a second kinematicbasic parameter or kinematic variable is measured in a second directionas a function of time at a second point or location of the ball gameracquet. The measurement rate during measuring the first and/or secondkinematic basic parameter is preferably at least 200 Hz. The measuredfirst kinematic basic parameter and the measured second kinematic basicparameter are then transformed into the frequency domain. Alternativelyor additionally, a linear combination of the measured first kinematicbasic parameter and the measured second kinematic basic parameter can betransformed into the frequency domain. The first and/or secondcoordinate of the point of contact is calculated on the basis of thetransformed kinematic basic parameter(s) in the frequency domain.

The transformation into the frequency domain can be carried out usingknown methods such as for example DFT, preferably FFT. The kinematicbasic parameter or kinematic variable can be the speed, theacceleration, or another kinematic basic parameter or kinematicvariable. Measurement is preferably carried out with an accelerationsensor and/or a gyrometer. Instead of the actually measured kinematicbasis parameter, a value derived therefrom can also be transformed. Forexample, the speed can be measured, the acceleration can be derived fromthat value and then the acceleration can be transformed into thefrequency domain and vice versa. The first and second coordinate of thepoint of contact refer to the coordinates within the plane of thestrings. Preferably, the first and second coordinates are perpendicularto each other. It is especially preferred that the first and secondcoordinates be oriented towards the x- and the y-coordinate,respectively.

Preferably, the first direction is essentially identical to the seconddirection. It is especially preferred that the first and seconddirections be essentially parallel to the z-coordinate. In other words,the speed or acceleration is preferably measured perpendicularly to thestrings or stringing plane of the ball game racquet.

The first point of the ball game racquet can be identical to the secondpoint of the ball game racquet. For example, the first kinematic basicparameter and the second kinematic basic parameter can be measured withone and the same sensor. However, preferably, the first point isdifferent from the second point. It is especially preferred that atleast one of the two points be located off to the side in relation tothe longitudinal axis of the ball game racquet.

Preferably, the calculation of the first and/or second coordinate of thepoint of contact on the basis of the transformed kinematic basicparameter(s) in the frequency domain comprises the following steps:Determining a characteristic frequency interval, determining at leastone characteristic value of the first and/or second kinematic basicparameter with respect to the characteristic frequency interval andcalculating the first and/or second coordinate of the point of contacton the basis of the at least one characteristic value. Thecharacteristic frequency interval is preferably determined or specifiedin advance. The lower limit of the characteristic frequency interval ispreferably between 0 Hz and 100 Hz, more preferred between 10 Hz and 80Hz and especially preferred between 25 Hz and 75 Hz. The upper limit ofthe characteristic frequency interval is preferably between 50 Hz and500 Hz, more preferred between 75 Hz and 400 Hz and especially preferredbetween 100 Hz and 300 Hz. According to this preferred embodiment of theprocess according to the invention, the point of contact is determinedon the basis of relatively small frequencies. Consequently, the processof the present invention does not require a high-resolution measurementin terms of time of the kinematic basic parameters. This allows the useof relatively simple standard sensors which are therefore reasonablypriced.

The characteristic value can preferably be one or a combination of thefollowing values: local or absolute minimum of the first and/or secondkinematic basic parameter in the characteristic frequency interval,local or absolute maximum of the first and/or second kinematic basicparameter in the characteristic frequency interval, mean value of thefirst and/or second kinematic basic parameter in the characteristicfrequency interval, mean value of the first and/or second kinematicbasic parameter in a subinterval of the characteristic frequencyinterval. According to the present invention, it has been found that thepoint of contact of the ball on the strings of the ball game racquetleaves a characteristic signature in the frequency domain of therespective kinematic basic parameter. Since this signature can havedifferent effects, the present invention is not limited to certaincharacteristic values. Rather, depending on the arrangement of thesensors and the vibration properties of the ball game racquet, differentcharacteristic values can be defined which directly correlate with thepoint of contact of the ball. Essentially, the present invention isbased, inter alia, on the basic idea that the frequency spectrumcorrelates with the point of contact of the ball on the strings of theball game racquet in different but specific ways. This correlation canbe found for every ball game racquet by means of correspondingexperiments. Once such a correlation is known, the first and/or secondcoordinate of the point of contact can be calculated by means of ananalysis of the spectrum in the frequency domain or a determination of acertain characteristic value of the kinematic basic parameter in thefrequency domain. This can for example be done by means of a tablewherein certain points of contact of the ball are assigned to certaincharacteristic values. However, the first and/or second coordinate ispreferably a function of one or more characteristic values.

According to a preferred embodiment, the first coordinate is thex-coordinate, the first direction is essentially parallel to thez-coordinate and the first point or location is provided at the handlesection. According to another preferred embodiment, the first coordinateis the y-coordinate, the first direction is essentially parallel to thez-coordinate and the first point is provided at the racquet head.According to another preferred embodiment, the first coordinate is thex-coordinate, the second coordinate is the y-coordinate and the firstand second directions are essentially parallel to the z-coordinate.Preferably, the first point is provided at the racquet head or thehandle section and the second point is provided at the racquet head.

The present invention is furthermore directed to a ball game racquetwith at least one first sensor for measuring at least one firstkinematic basic parameter or kinematic variable and a processing unit,wherein the first sensor and the processing unit are suitable forcarrying out the process as described above. Preferably, the ball gameracquet furthermore comprises a second sensor for measuring at least onesecond kinematic basic parameter. It is especially preferred that thefirst sensor be provided in or at the racquet head or handle section andthat the second sensor be provided in or at the racquet head.

The present invention is furthermore directed to a ball game racquetcomprising a racquet head accommodating strings, a handle section, anacceleration sensor and a processing unit suitable for calculating acoordinate of the point of contact of a ball on the strings based on theacceleration in a first direction measured by the acceleration sensor.Preferably, the acceleration sensor is provided in or at the handlesection. Preferably, the first direction runs along the longitudinalaxis of the racquet. Preferably, the ball game racquet comprises asecond acceleration sensor, wherein the processing unit is suitable forcalculating two coordinates of the point of contact of a ball on thestrings of the racquet based on the acceleration in the directionsmeasured by the two acceleration sensors, respectively.

Preferably, the processing unit is suitable for calculating twocoordinates of the point of contact of a ball on the strings of theracquet based on the acceleration in a first direction measured by theacceleration sensor.

Preferably, the ball game racquet furthermore comprises a gyrometerwherein the processing unit is suitable for calculating a secondcoordinate of the point of contact of a ball on the strings of theracquet based on the acceleration measured by the gyrometer. Thegyrometer is preferably provided in or at the handle section.

In the following, preferred embodiments of the present invention aredescribed in more detail with reference to the drawings.

FIGS. 1 a-c show the measuring result of an experiment;

FIG. 2 shows a flowchart of an exemplary algorithm for determining they-coordinate; and

FIG. 3 shows a flowchart of an exemplary algorithm for determining thex-coordinate.

FIGS. 1 a to 1 c show the result of an experiment which will be used toillustrate the basic idea of the present invention. In the drawing ofFIGS. 1 a and 1 b, a schematic of a tennis racquet is shown (for thisparticular experiment, the model “Extreme MP” from the company Head wasused) which has two sensors provided in the racquet head whose positionsare schematically indicated by an “x” and the designation HP1 and HP2.The sensors are acceleration sensors of the type “Bruel & Kjoer 4501”.The strings of the tennis racquet were hit with a hammer at definedpoints whereby the force is irrelevant since it can be normalized. Thepoints of contact (hitting points) of the hammer HP11 to HP19 are markedwith an “x” in the inserted sketch in FIGS. 1 a and 1 b. During theimpact and immediately after, the acceleration was measured by thesensors at the positions HP1 and HP2, respectively. The Fouriertransformed signal of the sensor at the position HP1 is shown as afunction of the frequency for the points of contact or impact HP11 toHP15 in FIG. 1 a. For the points of contact or impact HP13, HP17 andHP18, the corresponding signal is shown in FIG. 1 b. As can clearly beseen, the shapes of the various curves significantly differ from eachother depending on the point of contact or impact. For example, all thecurves show a minimum which occurs at distinctly different frequenciesdepending on the respective points of contact. In the case of alogarithmic scale, as shown for the curves of FIG. 1 a in FIG. 1 c,these minimum values are even more pronounced and it can clearly be seenhow the minimum shifts toward higher frequencies as the distance d ofthe point of contact or impact to the racquet handle increases.

The idea of the present invention is based on creating a correlationbetween the specific curve shape in the frequency domain and the actualpoint of contact of the ball on the strings. Once such a correlation hasbeen empirically established, the point of contact of the ball caneasily be determined by measuring the acceleration and transforming themeasured signal into the frequency domain. As is apparent from theexample of FIG. 1, fundamentally different characteristic values can bedefined for this purpose, on the basis of which the assignment can thenbe carried out. Thus, the curves in FIG. 1 not only differ in theposition of their minimum values but also for example in a differentlypronounced maximum or different amplitudes for example at 120 Hz. It istherefore emphasized that the examples of specific algorithms fordetermining the x- and/or y-coordinates of the point of contactdescribed below merely represent preferred embodiments and should not beregarded as limiting the present invention in any way. Rather, othercharacteristics of the various curves can be determined in the frequencydomain, by means of which conclusions can be drawn as to the position ofthe point of contact.

FIGS. 2 and 3 show a specific embodiment of a process according to thepresent invention for determining an x-coordinate and a y-coordinate.The inserted sketch in FIG. 2 shows a schematic ball game racquet with adefinition of the x- and y-coordinates whereby the origin of thecoordinate system is formed by the center of the strung area. At one ormore of the positions S₁, S₂ and S₃, an acceleration sensor can beprovided. However, the acceleration sensor S₃ is not necessary for thisparticular example. Only the acceleration sensors S₁ and S₂ are requiredwhich are preferably provided at the two arms or at the transitionbetween arm and bridge. Preferably, the acceleration sensors S₁ and S₂measure the acceleration over a time period of preferably 2 s with ameasuring rate of preferably 10,000 s⁻¹ along the z-direction, i.e.perpendicular to the x- and y-coordinate. The measured signal of theacceleration as a function of time of the two sensors S₁ and S₂ isschematically represented as S₁(t) and S₂(t), respectively, in FIGS. 2and 3. FIG. 2 shows a preferred flowchart for the determination of they-coordinate, while FIG. 3 shows a preferred flowchart for thedetermination of the x-coordinate.

For the determination of the y-coordinate shown as an example in FIG. 2,first the power spectral density (psd) of the measured signals S₁(t) andS₂(t) is calculated. In other words, a transformation of the measuredkinematic basic parameter into the frequency domain is carried out. Forthis purpose, a discrete Fourier transform such as for example FFT (fastFourier transform) can be used. The transformed signal is subsequentlyfiltered. The filtering can be carried out using known methods such asfor example a digital band pass filter (e.g. a third order Butterworthfilter). Then a characteristic value of the transformed signal iscalculated based on a characteristic frequency interval. In the exampleshown in the drawing, the characteristic frequency interval is [50 Hz,100 Hz] and the characteristic value is the mean value of thetransformed function in this frequency interval. If the thus determinedmean values of sensors S₁ and S₂ are designated S_(1y) and S_(2y),respectively, the y-coordinate of the point of contact can be determinedusing the following formula, wherein the values of S_(1y) and S_(2y) areto be entered with the unit m/s² and the result indicates they-coordinate in cm:

y=(S _(2y) −S _(1y))2.39

This formula was calculated heuristically for a certain tennis racquet.For a different type of racquet, the various numerical values of theabove formula can differ significantly from the example discussedherein. Furthermore, for a different type of racquet, it can beadvantageous to determine a different characteristic frequency intervaland/or a different characteristic value. FIG. 3 shows the correspondingalgorithm for the exemplary determination of the x-coordinate in aflowchart. In this example, the two measured signals S₁(t) and S₂(t) ofsensors S₁ and S₂ are first added and the resulting signal S(t) isconverted to a power spectral density S(f) using for example a discreteFourier transform (DFT). Then, an upper limit frequency f_(og) and alower limit frequency f_(ug) of the characteristic frequency interval[f_(ug), f_(og)] are determined. Preferably, the interval is [10 Hz, 200Hz]. On the basis of this characteristic frequency interval, the minimumof S(f) and the associated frequency f_(min) are then calculated. Thex-coordinate is then a function of the associated minimal frequencyf_(min): x=x(f_(min)). In a preferred embodiment, the x-coordinate ofthe point of contact can be determined using the following formula,wherein the frequency values are to be entered with the unit Hz and theresult indicates the x-coordinate in cm:

x=(f _(min)−150)/5.7, if f _(min)<170

x=(f _(min)−210)/10, if f _(min)>170

Alternatively, the x-coordinate can also be a function of the minimalfrequency and of the two frequencies of the characteristic frequencyinterval:

x=x(f _(min) ,f _(ug) ,f _(og))

As has already been mentioned repeatedly, these two embodiments arespecific examples which should by no means be regarded as limiting theinvention. Rather, using this example, it is merely demonstrated that itis possible to find a precise algorithm which assigns a coordinate ofthe point of contact to a kinematic basic parameter in the frequencydomain. However, this algorithm can basically be modified in many waysand adapted empirically to specific racquet geometries.

1. Process for determining at least one first coordinate of the point ofcontact of a ball on the strings of a ball game racquet with a racquethead and a handle section, wherein the longitudinal axis of the ballgame racquet defines an x-coordinate, the transverse axis of the ballgame racquet defines a y-coordinate and the line perpendicular to thestrings defines a z-coordinate, comprising the following steps: a)measuring at least one kinematic basic parameter in a first direction asa function of time at a first point of the ball game racquet, preferablywith a measurement rate of at least 200 Hz; b) transforming the measuredkinematic basic parameter into the frequency domain; and c) calculatingthe first coordinate of the point of contact on the basis of thetransformed kinematic basic parameter in the frequency domain. 2.Process for determining a first and a second coordinate of the point ofcontact of a ball on the strings of a ball game racquet with a racquethead and a handle section, wherein the longitudinal axis of the ballgame racquet defines an x-coordinate, the transverse axis of the ballgame racquet defines a y-coordinate and the line perpendicular to thestrings defines a z-coordinate, comprising the following steps: a)measuring a first kinematic basic parameter in a first direction as afunction of time at a first point of the ball game racquet, preferablywith a measurement rate of at least 200 Hz; b) measuring a secondkinematic basic parameter in a second direction as a function of time ata second point of the ball game racquet, preferably with a measurementrate of at least 200 Hz; c) transforming the measured first kinematicbasic parameter and the measured second kinematic basic parameter and/ora linear combination of the measured first and second basic parameterinto the frequency domain; and d) calculating the first and secondcoordinates of the point of contact on the basis of the transformedkinematic basic parameter(s) in the frequency domain.
 3. Processaccording to claim 2, wherein the first direction is essentiallyidentical to the second direction.
 4. Process according to claim 2,wherein the first point is different from the second point.
 5. Processaccording to claim 1, wherein the determination of the first and/orsecond coordinate of the point of contact on the basis of thetransformed kinematic basic parameter(s) in the frequency domaincomprises: a) determining a characteristic frequency interval; b)determining at least one characteristic value of the first and/or secondkinematic basic parameter with respect to the characteristic frequencyinterval; and c) calculating the first and/or second coordinate of thepoint of contact on the basis of the at least one characteristic value.6. Process according to claim 5, wherein the lower limit of thecharacteristic frequency interval is between 0 Hz and 100 Hz, preferablybetween 10 Hz and 80 Hz and especially preferred between 25 Hz and 75Hz.
 7. Process according to claim 5, wherein the upper limit of thecharacteristic frequency interval is between 50 Hz and 500 Hz,preferably between 75 Hz and 400 Hz and especially preferred between 100Hz and 300 Hz.
 8. Process according to claim 5, wherein thecharacteristic value comprises one or a combination of the followingvalues: local or absolute minimum of the first and/or second kinematicbasic parameter in the characteristic frequency interval, local orabsolute maximum of the first and/or second kinematic basic parameter inthe characteristic frequency interval, mean value of the first and/orsecond kinematic basic parameter in the characteristic frequencyinterval, mean value of the first and/or second kinematic basicparameter in a subinterval of the characteristic frequency interval. 9.Process according claim 5, wherein the first and/or second coordinate isa function of the characteristic value.
 10. Process according to claim1, wherein the first coordinate is the x-coordinate, the first directionis essentially parallel to the z-coordinate and the first point isprovided at the handle section.
 11. Process according to claim 1,wherein the first coordinate is the y-coordinate, the first direction isessentially parallel to the z-coordinate and the first point is providedat the racquet head.
 12. Process according to claim 1, wherein the firstcoordinate is the x-coordinate, the second coordinate is they-coordinate and the first and second directions are essentiallyparallel to the z-coordinate.
 13. Process according to claim 12, whereinthe first point is provided at the racquet head or at the handle sectionand the second point is provided at the racquet head.
 14. Processaccording to claim 1, wherein the first and/or second kinematic basicparameter is acceleration.
 15. Ball game racquet comprising a racquethead, a handle section, at least one first sensor for measuring at leastone first kinematic basic parameter and a processing unit, wherein thefirst sensor and the processing unit are suitable for carrying out theprocess according to claim
 1. 16. Ball game racquet according to claim15, furthermore comprising a second sensor for measuring at least onesecond kinematic basic parameter.
 17. Ball game racquet according toclaim 15, wherein the first sensor is provided in or at the racquet heador handle section and wherein the second sensor is provided in or at theracquet head.
 18. Ball game racquet comprising a racquet headaccommodating strings, a handle section, an acceleration sensor and aprocessing unit suitable for calculating a coordinate of the point ofcontact of a ball on the strings of the racquet based on theacceleration in a first direction measured by the acceleration sensor.19. Ball game racquet according to claim 18, wherein the accelerationsensor is provided in or at the handle section.
 20. Ball game racquetaccording to claim 18, wherein the first direction runs along thelongitudinal axis of the racquet.
 21. Ball game racquet according toclaim 18, furthermore comprising a second acceleration sensor, whereinthe processing unit is suitable for calculating two coordinates of thepoint of contact of a ball on the strings of the racquet based on theacceleration in two directions measured by the two acceleration sensors,respectively.
 22. Ball game racquet according to claim 18, wherein theprocessing unit is suitable for calculating two coordinates of the pointof contact of a ball on the strings of the racquet based on theacceleration in a first direction measured by the acceleration sensor.23. Ball game racquet according to claim 18, furthermore comprising agyro sensor wherein the processing unit is suitable for calculating asecond coordinate of the point of contact of a ball on the strings ofthe racquet based on the acceleration measured by the gyro sensor. 24.Ball game racquet according to claim 23, wherein the gyro sensor isprovided in or at the handle section.
 25. Process according to claim 2,wherein the determination of the first and/or second coordinate of thepoint of contact on the basis of the transformed kinematic basicparameter(s) in the frequency domain comprises: a) determining acharacteristic frequency interval; b) determining at least onecharacteristic value of the first and/or second kinematic basicparameter with respect to the characteristic frequency interval; and c)calculating the first and/or second coordinate of the point of contacton the basis of the at least one characteristic value.
 26. Processaccording to claim 2, wherein the first coordinate is the x-coordinate,the first direction is essentially parallel to the z-coordinate and thefirst point is provided at the handle section.
 27. Process according toclaim 2, wherein the first coordinate is the y-coordinate, the firstdirection is essentially parallel to the z-coordinate and the firstpoint is provided at the racquet head.
 28. Process according to claim 2,wherein the first coordinate is the x-coordinate, the second coordinateis the y-coordinate and the first and second directions are essentiallyparallel to the z-coordinate.
 29. Process according to claim 2, whereinthe first and/or second kinematic basic parameter is acceleration.